The average height of the highest third of the waves, the wave height that forecasts give and an experienced eye estimates.
Significant wave height (Hs, also H⅓) is the average height of the highest third of the waves in a sea state, measured from trough to crest. It's the number wave forecasts and buoys report, and it comes close to what an experienced sailor would estimate by eye, because the eye notices the big waves and overlooks the small ones.
What it means for single waves
Hs is an average, not a maximum. In a sea with a significant wave height of 2 m:
- many waves are smaller than 2 m,
- waves around 1.5 times Hs, here 3 m, turn up regularly,
- roughly one wave in a few thousand reaches about twice Hs, here 4 m.
Over a long day at sea, you should expect to meet some waves well above the forecast figure.
Wind sea and swell combined
Forecasts often list wind sea and swell separately. The combined significant wave height is not their sum but:
Hs total = √(Hs wind sea² + Hs swell²)
A 1.5 m wind sea on a 2 m swell gives about 2.5 m, not 3.5 m. How uncomfortable a given height feels depends heavily on the wave period. See also the Douglas sea scale and the guide sea state and waves.
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